1. Field of the Invention
The present invention relates to the production of printing plates from photopolymerizable resins. More particularly, this invention relates to the production of printing plates wherein a cap resin layer with a consistent thickness is automatically laid on a glass plate-negative assembly irrespective of the thickness of the photopolymerizable resin layer on the assembly.
2. Brief Description of the Art
The production of printing plates by selectively cross-linking polymerizable resins is known in the prior art. The printing plate is conventionally formed by first placing a negative on a supporting glass plate. An optically transparent capping resin layer is then deposited on top of the negative and the capping resin layer is subsequently coated with a layer of photopolymerizable resin. A backing sheet is then placed on top of the photopolymerizable resin, and the backing sheet is then covered by another glass sheet. Irradiation by actinic light, such as UV light, through the top glass/backing sheet combination forms a solid floor layer of photoresin, which adheres to the backing sheet. The thickness of the floor layer is less than the total thickness of the photoresin. Irradiation through the lower glass plate/negative/capping resin layer selectively hardens the photoresin to form an image-printing surface which mirrors the image on the negative. The hardened regions adhere to the floor layer, but not to the transparent release sheet. Subsequent processing removes unhardened (liquid) photoresin to reveal a relief image. The photopolymerizable resin layer which is placed on the release film will have various thicknesses depending on the thickness of the printing plate to be produced, while the capping resin layer has a relatively constant thickness which is a fraction of the thickness of the cross-linkable resin layer. Both of the resin layers are deposited on the glass plate assembly from a carriage which carries bulk supplies of each of the cap and cross-linkable resins, and which is selectively passed over the glass plate-negative assembly so as to facilitate deposition of the resin layers on the negative. The thickness of the capping resin layer is controlled by a doctor blade which is mounted on the carriage and is drawn across the capping resin layer so as to level and control the thickness of the capping resin layer on the negative. The thickness of the cross-linkable resin layer is determined by the distance of the carriage from the glass plate assembly. Thus, when a relatively thin layer of the cross-linkable resin is to be deposited on the glass plate assembly, the carriage will be lowered toward the glass plate assembly, and when a relatively thick layer of the cross-linkable resin is to be deposited on the glass plate assembly, the carriage will be lifted away from the glass plate assembly.
The resultant assembly will have a relatively thin and constant thickness cap resin layer formed on the supporting glass plate assembly; and can have either relatively thick or relatively thin cross-linkable resin layers on them. The prior art procedure is satisfactory when the printing plates being produced have the same thickness from production run-to-run, and thus the same thickness cross-linkable resin layer; however, when different thickness printing plates are produced from run-to-run, a manual adjustment of the position of the cap resin doctor blade must be made relative to the glass support plate so as to provide the proper cap resin layer thickness. Presently, when the thickness of the resin printing plate is changed, the cap resin layer must be manually poured onto the glass support plate assembly; and the position of the capping blade must then be manually adjusted relative to the glass support plate assembly; and the capping blade must then be manually dragged across the resin puddle, and readjusted if necessary, until one can confirm that the resultant cap resin layer thickness will be as required. The aforesaid procedure is both time-consuming and wasteful of resin.
It would be desirable to be able to automatically produce the constant desired thickness of the cap resin layer irregardless of the thickness of the printing plates being produced.